Butadiene (1,3-butadiene) is an important base chemical and is used, for example, for production of synthetic rubbers (butadiene homopolymers, styrene-butadiene rubber or nitrile rubber) or for production of thermoplastic terpolymers (acrylonitrile-butadiene-styrene copolymers). Butadiene is also converted to sulfolane, chloroprene and 1,4-hexamethylenediamine (via 1,4-dichlorobutene and adiponitrile). Through dimerization of butadiene, it is also possible to obtain vinylcyclohexene, which can be dehydrogenated to styrene.
Butadiene can be prepared by thermal cracking (steamcracking) of saturated hydrocarbons, typically proceeding from naphtha as the raw material. The steamcracking of naphtha affords a hydrocarbon mixture of methane, ethane, ethene, acetylene, propane, propene, propyne, allene, butanes, butenes, butadiene, butynes, methylallene, and C5 and higher hydrocarbons.
Butadiene can also be obtained by the oxidative dehydrogenation of n-butenes (1-butene and/or 2-butene) in the presence of molecular oxygen. The input gas stream utilized for the oxidative dehydrogenation (oxydehydrogenation, ODH) of n-butenes to butadiene may be any desired mixture comprising n-butenes. For example, it is possible to use a fraction which comprises n-butenes (1-butene and/or 2-butene) as the main constituent and has been obtained from the C4 fraction from a naphtha cracker by removing butadiene and isobutene. In addition, it is also possible to use gas mixtures which comprise 1-butene, cis-2-butene, trans-2-butene or mixtures thereof and have been obtained by dimerization of ethylene as the input gas stream. In addition, the input gas streams used may be gas mixtures which comprise n-butenes and have been obtained by catalytic fluidized bed cracking (fluid catalytic cracking, FCC).
As well as n-butenes and molecular oxygen, the reaction gas mixture generally comprises inert components. “Inert components” means here that they are converted to an extent of less than 90% under the reaction conditions of the ODH. Inert components are, for example, steam and nitrogen, but also, for example, alkanes such as methane. The molar ratio of the inert component to molecular oxygen here is generally higher than is present in air, particularly in order to avoid the risk of explosions. This can be done, for example, by using air as the oxygenous gas and diluting it with molecular nitrogen. However, the provision of large volumes of concentrated nitrogen is costly and disadvantageous from an economic point of view. In addition, this can be done by using molecular oxygen-depleted air (lean air) as the oxygenous gas. In addition, this can be done by diluting air with lean air.
Processes for oxidative dehydrogenation of butenes to butadiene are known in principle.
US 2012/0130137A1, for example, describes a process of this kind using catalysts comprising oxides of molybdenum, bismuth and generally further metals. For the lasting activity of such catalysts for the oxidative dehydrogenation, a critical minimum level of partial oxygen pressure is required in the gas atmosphere in order to avoid an excessive reduction and hence loss of performance of the catalysts. For this reason, it is generally also not possible to work with a stoichiometric oxygen input or complete oxygen conversion in the oxydehydrogenation reactor (ODH reactor). US 2012/0130137 describes, for example, an oxygen content of 2.5 to 8% by volume in the starting gas.
The N2/O2 ratio in the reaction gas mixture is set to the desired value by diluting air as the oxygenous gas with nitrogen gas.
The need for an oxygen excess for such catalyst systems is common knowledge and is reflected in the process conditions when catalysts of this kind are used. Representative examples include the comparatively recent studies by Jung at al. (Catal. Surv. Asia 2009, 13, 78-93; DOI 10.1007/s10563-009-9069-5 and Applied Catalysis A: General 2007, 317, 244-249; DOI 10.1016/j.apcata.2006.10.021).
The presence of oxygen alongside butadiene downstream of the ODH reactor stage, in the workup section of such processes operated with an excess of oxygen, however, is afflicted with risks. Especially in the liquid phase, the formation and accumulation of organic peroxides should be examined. These risks have been discussed, for example, by D. S. Alexander (Industrial and Engineering Chemistry 1959, 51, 733-738).
JP 2011-006381 A to Mitsubishi addresses the risk of peroxide formation in the workup section of a process for preparing conjugated alkadienes. As a solution, the addition of polymerization inhibitors to the absorption solutions for the process gases and the setting of a maximum peroxide content of 100 ppm by weight by heating the absorption solutions is described. However, there is no information as to avoidance or monitoring of peroxides in upstream process steps. A particularly critical aspect is the step of cooling the ODH reactor output with a water quench. Organic peroxides formed are barely soluble in water, and so they are deposited and can accumulate in the apparatus in solid or liquid form, instead of being discharged with the aqueous purge stream from the quench. At the same time, the temperature of the water quench is not so high that sufficiently high and constant breakdown of the peroxides formed can be assumed
The catalytic oxidative dehydrogenation can form high-boiling secondary components, for example maleic anhydride, phthalic anhydride, benzaldehyde, benzoic acid, ethylbenzene, styrene, fluorenone, anthraquinone and others. Such deposits can lead to blockages and to a rise in the pressure drop in the reactor or beyond the reactor in the workup area, and thus disrupt regulated operation. Deposits of the high-boiling secondary components mentioned can also impair the function of heat exchangers or damage moving apparatuses such as compressors. Steam-volatile compounds such as fluorenone can get through a quench apparatus operated with water and precipitate beyond it in the gas discharge lines. In principle, there is therefore also the risk that solid deposits will get into downstream apparatus parts, for example compressors, and cause damage there.
US 2012/0130137A1 paragraph [0122] also refers to the problem of high-boiling by-products. Particular mention is made of phthalic anhydride, anthraquinone and fluorenone, which are said to be present typically in concentrations of 0.001 to 0.10% by volume in the product gas. US 2012/0130137A1 paragraphs [0124]-[0126] recommends cooling the hot reactor discharge gases directly, by contact with a cooling liquid (quench tower), at first to typically 5 to 100° C. The cooling liquids mentioned are water or aqueous alkali solutions. There is explicit mention of the problem of blockages in the quench by high boilers from the product gas or by polymerization products of high-boiling, by-products from the product gas, and for this reason it is said to be advantageous that high-boiling by-products are entrained as little as possible from the reaction section to the cooling section (quench).
KR 2013-0036467 and KR 2013-0038468 likewise recommend cooling the hot reactor discharge gases directly by contact with a coolant. The coolants used are water-soluble organic coolants, in order to better cool the secondary component.
JP 2011-001341A describes a two-stage cooling operation for a process for oxidative dehydrogenation of alkenes to conjugated alkadienes. This involves first setting the product discharge gas from the oxidative dehydrogenation to a temperature between 300 and 221° C. and then cooling it further to a temperature between 99 and 21° C. Paragraphs [0066] ff. state that the temperature between 300 and 221° C. is preferably established using heat exchangers, but a portion of the high boilers could also precipitate out of the product gas in these heat exchangers. JP 2011-001341A therefore describes occasional washing of deposits out of the heat exchangers with organic or aqueous solvents. Solvents described are, for example, aromatic hydrocarbons such as toluene or xylene, or an alkaline aqueous solvent, for example the aqueous solution of sodium hydroxide. In order to avoid excessive frequency of shutdown of the process to clean the heat exchanger, JP2011-001341A describes a setup having two heat exchangers arranged in parallel, which are each alternately operated or washed (called A/B operation mode).
JP 2010-90083 A describes a process for oxidative dehydrogenation of n-butenes to butadiene, in which the product gas of the oxidative dehydrogenation is cooled and dewatered. Subsequently, butadiene and unconverted butenes and butane are absorbed in a solvent from the C4-containing input gas stream. The residual gas which has not been absorbed by the solvent is subsequently disposed of by incineration. If a solvent such as toluene having a low boiling point is used as the absorbent, this is recovered from the residual gas stream by absorption in a solvent having a high boiling point for example decane, for the purpose of avoidance of solvent losses.
JP 2012072086 states, in paragraph [0014], that a gas in which the hydrocarbons, such as butadiene, n-butene, n-butane, isobutane, have been removed from the product gas mixture can be recycled into the oxydehydrogenation as an oxygenous gas. The document does not make any statements about how such a recycle gas stream is obtained, and which impurities are present therein.